(a) Field of the Invention
The present invention relates to a polypropylene resin composition, the molded article of the resin composition having excellent adhesion to paints or varnishes without any treatment after molding such as an etching treatment with vapor of an organic solvent, e.g., trichloroethane, a plasma treatment and a flame treatment for oxidizing the surface thereof, as well as a method for the manufacturing a coated molded article of the resin composition.
(b) Description of the Prior Art
Metal materials for external plate materials such as bumpers and fenders for automobiles have increasingly been superseded by polypropylene resin compositions. However, regarding the coating of the molded articles of polypropylenes, they must usually be coated after subjecting it to an appropriate pretreatment since polypropylene per se has extremely low coating properties. As such pretreatments, there have long been used, for instance, an etching treatment with vapor of an organic solvent such as trichloroethane (1,1,1-trichloroethane), a plasma treatment and a flame treatment for oxidizing the surface thereof. In case of large size molded articles such as bumpers for automobiles, the optimum method for coating the same comprises pretreating the surface of the articles, in particular, etching the surface with vapor of trichloroethane, then applying a primer layer containing, for instance, a chlorinated polyolefin in advance, drying it by heating, applying a paint or varnish (hereinafter simply referred to as "paint(s)") and then heating to harden the coated paint layer since the method requires the use of a simple apparatus and allows the curtailment of the coating expenses. Incidentally, highly volatile, chemically stable chlorine atom-containing compounds such as trichloroethane are hardly decomposed after being released in the air and if they diffuse to the ozone layer surrounding the earth, ozone which serves to absorb ultraviolet rays or the like emitted by the sun would be severely decomposed. Accordingly, it has been believed that the decomposition of ozone possibly leads to an increase in the quantity of harmful ultraviolet rays or the like which are pouring on the ground and that the frequency of developing diseases such as cutaneous cancer would, in turn, be increased. With these points as background, it has been required for the reduction of the amount of chlorine atom-containing organic solvents such as trichloroethane on a worldwise scale and the use thereof must be wholly abolished in the near future. For instance, in the 2nd Meeting for the Agreement of Montreal Protocol held on June, 1990, it has been decided that the use and preparation of trichloroethane should be wholly abolished by 2005 and accordingly the use and preparation thereof have stepwise been reduced until the deadline. For this reason, trichloroethane can no longer be used in the pretreatment of large size molded articles such as bumpers for automobiles. However, if other pretreatments in which trichloroethane is not used, for instance, the plasma treatment is employed instead, it is necessary to built a big installation for treating large-size molded articles such as bumpers for automobiles and too much expenses are in turn required. On the other hand, when a combustible resin is subjected to a flame treatment, various problems arise. For instance, thin portions or the like thereof may be molten and may begin to burn. Therefore, the yield of coated article is substantially impaired. Further, the size of the oxidizing flame for oxidizing the surface of polypropylene cannot be increased too much. If a material to be treated has a complicated shape, a part thereof is incompletely treated and this leads to the formation of portions having extremely low adhesion to a coating film subsequently applied thereto. Thus, the flame treatment can only be applied to materials having approximately flat and simple shapes.
On the other hand, there has been proposed the use of block copolymers of styrene and conjugated dienes or hydrogenated derivatives thereof as impact modifiers for styrene resins and polyolefinic resins (see, for instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") Nos. Sho 58-215446, Sho 60 133039, Sho 61-014248 and Hei 1-174550). However, currently used styrene/butadiene block copolymers, styrene/isoprene copolymers or hydrogenated derivatives thereof have high viscosities and, therefore, if they are mechanically kneaded together with polypropylene and then injection-molded, the rubber component is present in the superficial polypropylene layer of the resulting molded article in approximately spherical shape. For this reason, the incorporation of these rubber components does not contribute to the improvement in the coating properties even if a solvent included in a paint penetrates into the superficial portion of the article because the surface area of the rubber layer on the article is not so large. Furthermore, it has bad flowability, results in flow marks and/or jetting on injection molding and the molded articles have low surface gloss. Moreover, J. P. KOKAI No. Hei 3-277636 discloses a composition which comprises a polypropylene and a hydrogenated styrene/isoprene block copolymer or comprises a styrene/butadiene block copolymer, talc and a hydrogenated product of a diene polymer having terminal hydroxyl groups. The composition can provide a molded article which has excellent coating properties without treatment with trichloroethane vapor and to which a paint can directly be applied. The patent further discloses that the hydrogenated styrene/isoprene block copolymer or the styrene/butadiene block copolymer should have a melt index of not more than 20 g/10 min as determined at a temperature of 230.degree. C. and a load of 2.16 kg/cm.sup.2 for obtaining improved coating properties and low temperature impact resistance.
On the other hand, the environmental pollution due to the exhaust gas released from automobiles has become a problem in the United States and Europe. Accordingly, as one of the measures therefor, it would be institutionalized, in the United States, that the automobiles must be made lighter and the combustion efficiency is further improved to thus reduce the amount of exhaust gas released in the air. To this end, it has been desired that the specific gravity of polypropylene resin compositions or the like used in various parts of automobiles should be reduced to a level of several percents lower than that of the conventional materials. A preferred means for making, lighter, the bumpers or the like, which comprise fillers such as talc in an amount ranging from about 5 to 20%, for automobiles is to reduce the amounts of fillers having specific gravities greater than that of resins. However, if the amounts of fillers are reduced, the mold shrinkage factor and linear expansion coefficient of the resulting resin compositions are greater than those of the conventional materials containing sufficient amounts of fillers and thus the resulting materials are not suitable for use in making large size molded articles.